The present invention generally relates to rotation control apparatuses for motors, and more particularly to a rotation control apparatus for controlling the rotation of a motor for rotating or moving a recording medium by use of a rotation detection signal having a period dependent on the rotational speed of the motor when the motor starts to rotate and by use of a synchronizing signal reproduced from the recording medium when the motor reaches a steady state rotational speed.
The rotation of a motor for rotating or moving a recording medium such as a video disc which is pre-recorded with a video signal, is generally controlled based on a phase error voltage V.sub.1 which is obtained by comparing the phase of a first reference signal produced from an oscillator with the phase of a horizontal synchronizing signal within the video signal which is reproduced from the recording medium. However, a synchronizing signal separating circuit which is used to separate the reproduced horizontal synchronizing signal from the reproduced video signal, can only separate the reproduced horizontal synchronizing signal when the reproduced horizontal synchronizing signal within the reproduced video signal has a frequency which is within a predetermined frequency range having a regular frequency f.sub.H of the horizontal synchronizing signal as the center of the predetermined frequency range. Hence, during a time period from the start of the motor until the frequency of the reproduced horizontal synchronizing signal reaches a frequency within the predetermined frequency range, a reproduced horizontal synchronizing signal cannot be obtained from the synchronizing signal separating circuit.
Accordingly, during the above time period, the rotation of the motor is controlled based on a phase error voltage V.sub.2 which is obtained by comparing the phase of an output signal of a frequency generator which generates a signal having a frequency proportional to the rotational speed of the motor with the phase of a second reference signal which is obtained by frequency-dividing the first reference signal in a frequency divider. The phase error voltage V.sub.2 is applied to the motor through a phase compensating circuit and a motor driving circuit, and controls the motor so that the rotational speed of the motor swiftly increases to a rotational speed which is near the steady state rotational speed. When the rotational speed of the motor reaches a rotational speed near the steady state rotational speed and the reproduced horizontal synchronizing signal is obtained from the synchronizing signal separating circuit, the phase error voltage V.sub.1 is obtained by attenuating a high frequency component of a phase error voltage which is obtained by comparing the phase of the first reference signal and the phase of the reproduced horizontal synchronizing signal in a filter circuit, and this phase error voltage V.sub.1 is supplied to a switching circuit. The switching circuit switches the output phase error voltage from the phase error voltage V.sub.2 to the phase error voltage V.sub.1, responsive to an output signal of a discriminating circuit which discriminates that the rotational speed of the motor has reached a rotational speed near the steady state rotational speed and to an output signal of a detector which detects that the recording medium is in a playable state. The output phase error voltage of the switching circuit is applied to the motor through the phase compensating circuit and the motor driving circuit. As a result, the rotational speed of the motor is controlled so that the phase of the first reference signal coincides with the phase of the reproduced horizontal synchronizing signal.
When the control voltage for controlling the rotational speed of the motor is switched over from the phase error voltage V.sub.2 to the phase error voltage V.sub.1 in the conventional rotation control apparatus, measures must be taken so that the stability of the control operation of the rotation control apparatus is satisfactory upon taking place of a lock-in (pull-in) in which the reproduced horizontal synchronizing signal is locked with the first reference signal. The filter circuit described before comprises a first resistor which is coupled between an input terminal and an output terminal, and a second resistor and a capacitor coupled in series between the first resistor and the output terminal. The stability with which the rotation control apparatus carries out the control operation, is greatly affected by a terminal voltage V.sub.c-c of the capacitor at a time before the lock-in takes place.
In a case where there is a great difference between the terminal voltage V.sub.c-c described before and a terminal voltage V.sub.c-s of the capacitor after the lock-in takes place and the rotation of the motor has reached the steady state rotational speed, the control system exceeds a linear operating range due to the charging and discharging of the capacitor that takes place when the switching circuit is switched. As a result, the stability of the control operation carried out by the rotation control apparatus becomes poor, and in an extreme case, the rotation control apparatus can no longer carry out the control operation. The terminal voltage V.sub.c-s changes when the load of the motor changes due to a change in the ambient temperature or the like and when a drift occurs in the output signal of the motor driving circuit. Therefore, even in a case where the terminal voltage V.sub.c-c is initially set to an ideal voltage, there is a problem in that the lock-in may become unstable due to causes such as a change in the operating condition of the motor.